Abstract

Introduced species are having major impacts in terrestrial, freshwater and marine ecosystems worldwide.
It is increasingly recognised that effects of multiple
species often cannot be predicted from the effect of each
species alone, due to complex interactions, but most
investigations of invasion impacts have examined only
one non-native species at a time and have not addressed
the interactive effects of multiple species. We conducted
a field experiment to compare the individual and combined
effects of two introduced marine predators, the
northern Pacific seastar Asterias amurensis and the
European green crab Carcinus maenas, on a soft-sediment
invertebrate assemblage in Tasmania. Spatial
overlap in the distribution of these invaders is just
beginning in Tasmania, and appears imminent as their respective ranges expand, suggesting a strong overlap in
food resources will result from the shared proclivity for
bivalve prey. A. amurensis and C. maenas provide good
models to test the interaction between multiple introduced
predators, because they leave clear predator-specific traces of their predatory activity for a number of
common prey taxa (bivalves and gastropods). Our
experiments demonstrate that both predators had a
major effect on the abundance of bivalves, reducing
populations of the commercial bivalves Fulvia tenuicostata
and Katelysia rhytiphora. The interaction between
C. maenas and A. amurensis appears to be one of resource
competition, resulting in partitioning of bivalves
according to size between predators, with A. amurensis
consuming the large and C. maenas the small bivalves.
At a large spatial scale, we predict that the combined
effect on bivalves may be greater than that due to each
predator alone simply because their combined distribution
is likely to cover a broader range of habitats. At a
smaller scale, in the shallow subtidal, where spatial
overlap is expected to be most extensive, our results
indicate the individual effects of each predator are likely
to be modified in the presence of the other as densities
increase. These results further highlight the need to
consider the interactive effects of introduced species,
especially with continued increases in the number of
established invasions.